Oxygen permeable electrode system

ABSTRACT

An electrochemical device ( 18 ) for generating a desired gas of the type includes an ionically conductive electrolyte layer ( 20 ), a porous electrode layer ( 22 ), and a current collector layer ( 16 ) that has a high electrical conductivity and is porous to a desired gas ( 24 ) generated by the electrochemical device ( 18 ). The current collector layer ( 16 ) is substantially formed as a film comprised of a layer of spherical refractory material objects ( 26 ) having a conductive coating ( 12 ) of a precious metal. The coated spherical objects ( 26 ) have a desired diameter ( 28 ) making them suitable for forming into the film.

CROSS REFERENCE TO RELATED APPLICATIONS

[0001] This application claims the benefit of U.S. ProvisionalApplication Serial No. 60/319,661, filed Oct. 31, 2002, entitled OXYGENPERMEABLE ELECTRODE SYSTEM.

BACKGROUND OF INVENTION

[0002] 1. Technical Field

[0003] The invention relates to the field of electrochemical gasgenerators, and more particularly for an electrode composition andmethod for forming electrodes for ceramic oxygen generating systems(COGS).

[0004] 2. Background Art

[0005] Electrochemical devices, such as fuel cells, oxygen sensors, andoxygen separation membranes are becoming increasingly popular ascandidates for a variety of applications. Elevated operatingtemperatures required for efficient operation of these devices pose amajor challenge for achieving required cost, power efficiency, anddevice longevity for prospective markets.

[0006] Common failure modes of the electrodes or current collectors inoxygen separation membrane systems include:

[0007] a. Densification of porous electrode and current collectorcoatings during operation;

[0008] b. Chemical attack of coatings by atmospheric contaminants; and,

[0009] c. Delamination of coatings from each other and from thesubstrate.

[0010] Typical current collectors which operate in an oxidizingatmosphere at temperatures exceeding 600° C. include Silver (Ag), Gold(Au), Platinum (Pt), Palladium (Pd), and alloys of these constituents.Although the combination of high electrical conductivity and low costmakes Silver (Ag) an attractive material, its tendency to sinter andvaporize under typical operating temperatures limits the applicabilityof the material in its pure form. Silver is also vulnerable to chemicalattack in some conditions that might be encountered in the case ofchemical warfare or other contaminated environments, as well asoxidation when in contact with high pressure, high purity oxygen.Inclusion of other precious metals, and their alloys, lead to a dramaticreduction in conductivity and increase in cost.

[0011] Examples of such known electrodes used with electrochemical gasgenerating systems are disclosed in U.S. Pat. No. 5,378,345, issued Jan.3, 1995; U.S. Pat. No. 5,985,113, issued Nov. 16, 1999; U.S. Pat. No.5,972,182, issued Oct. 26, 1999; U.S. Pat. No. 6,194,335, issued Feb.27, 2001; U.S. Pat. No. 6,203,676, issued Mar. 20, 2001; and U.S. Pat.No. 6,290,757, issued Sep. 18, 2001.

[0012] Delamination between precious metal coatings and other coatingsor substrates is common due to the relatively higher thermal expansionof the metal coating versus the other materials, typically ceramic, usedin electrochemical devices.

[0013] The present invention includes an electrode and current collectorsystem that is resistant to the aforementioned failure modes, whileminimizing the use of precious metals, thereby enhancing overallelectrochemical system performance and reducing cost as compared totraditional or known approaches.

[0014] While the above cited references introduce and disclose a numberof noteworthy advances and technological improvements within the art,none completely fulfills the specific objectives achieved by thisinvention.

SUMMARY OF INVENTION

[0015] In accordance with the present invention, a current collector foran electrochemical device of the type that includes an ionicallyconductive electrolyte layer for gas generating includes the currentcollector layer having a high electrical conductivity and being porousto desired gas generated by the electrochemical device. The currentcollector layer is substantially formed as a film comprised of a layerof spherical refractory material objects having a conductive coating ofa precious metal. The coated spherical objects have a desired diametermaking them suitable for forming into the electrically conductive film.

[0016] These and other objects, advantages and features of thisinvention will be apparent from the following description taken withreference to the accompanying drawings, wherein is shown the preferredembodiments of the invention.

BRIEF DESCRIPTION OF DRAWINGS

[0017] A more particular description of the invention briefly summarizedabove is available from the exemplary embodiments illustrated in thedrawing and discussed in further detail below. Through this reference,it can be seen how the above cited features, as well as others that willbecome apparent, are obtained and can be understood in detail. Thedrawings nevertheless illustrate only typical, preferred embodiments ofthe invention and are not to be considered limiting of its scope as theinvention may admit to other equally effective embodiments.

[0018]FIG. 1 is an illustration of a cross section of a currentcollector composed of conductive coating applied to refractoryparticles.

[0019]FIG. 2 is an elevational view of a simplified electrochemicaldevice of the present invention.

DETAILED DESCRIPTION

[0020] So that the manner in which the above recited features,advantages, and objects of the present invention are attained can beunderstood in detail, more particular description of the invention,briefly summarized above, may be had by reference to the embodimentthereof that is illustrated in the appended drawings. In all thedrawings, identical numbers represent the same elements.

[0021] The current collector 16 of the present invention utilizes anouter conductive precious metal coating 12 uniformly applied to theexterior of refractory particles or spheres 14 as shown in FIG. 1.

[0022] Referring now to FIG. 2 the electrochemical device 18 forgenerating a desired gas of the type includes an ionically conductiveelectrolyte layer 20, a porous electrode layer 22, and the currentcollector layer 16 that has a high electrical conductivity and is porousto desired gas 24 generated by the electrochemical device 18. Thecurrent collector layer 16 is substantially formed as a film comprisedof a layer of spherical refractory material objects 26 having aconductive coating 12 of a precious metal. The coated spherical objects26 have a desired diameter 28 making them suitable for forming into theelectrically conductive film.

[0023] The precious metal is from the aforementioned set of materialsAg, Au, Pt, Pd, and corresponding alloys. The refractory materialtypically is a ceramic, such as zirconia, alumina, or ceria, but couldalso be a perovskite or other conductive ceramic with a similar thermalexpansion coefficient to oxygen conducting ceramics (that is zirconiaand ceria).

[0024] Uniformity of the spheres 26 shown in FIG. 1 is important tomaximizing the porosity of a layer formed from the coated spheres 26.

[0025] The coated spherical objects 26 generally have a uniform diameterthat is selected to be appropriate for their formation in the filmforming the current collector 16.

[0026]FIG. 2 shows a cross section of a typical electrochemical device18 in which the bottom layer 20 is an ionically conductive electrolyte,the middle layer 22 is a porous electrode with high electricalconductivity and catalytic activity for the ion species transported bythe electrolyte layer 20. The upper layer 16 is the porous currentcollector that preferably has a very high electrical conductivity.Silver (Ag) has the highest electrical conductivity for all knownmaterials that can meet the current collector layer 16 operatingrequirements.

[0027] At the interface between the electrolyte and electrode layers 20and 22, the primary function is to ensure the efficient transport ofoxygen or other selected gas ions into and out of the electrolyte 20.While adhesion of the electrolyte and the electrode layers 20 and 22 isimportant, it is also important to maintain a high surface area foroxygen or other gas ions to be catalyzed.

[0028] In the present electrochemical electrode composition, the currentcollector layer 16 is formed from the conductive coated refractoryparticles 26 shown in FIG. 1. A desired permeability or porosity of thecurrent collector layer 16 is about 50 percent. A continuous electricalpath is formed through the current conductor layer due to the conductivecoated refractory particles touching one another 30 or abutting againstone another.

[0029] The electrical performance of the disclosed conductive coating isexpected to be similar to a coating containing solid conductive spheresdue to the fact that the charge distribution in a conductive sphere isconcentrated at the perimeter of the sphere.

[0030] The foregoing disclosure and description of the invention areillustrative and explanatory thereof, and various changes in the size,shape and materials, as well as in the details of the illustratedconstruction may be made without departing from the spirit of theinvention.

1. A current collector for an electrochemical device of the type thatincludes an ionically conductive electrolyte layer for gas generating,the invention comprising: the current collector layer having a highelectrical conductivity and being porous to desired gas generated by theelectrochemical device; and, the current collector layer beingsubstantially formed as a film comprised of a layer of sphericalrefractory material objects having a conductive coating of a preciousmetal.
 2. The invention of claim [claim Reference] wherein the preciousmetal is selected from the group consisting of silver, palladium, gold,or platinum, or alloys of these metals with other metals.
 3. Theinvention of claim [claim Reference] wherein the refractory material isa ceramic.
 4. The invention of claim [claim Reference] wherein therefractory material is selected from the group consisting of zirconia,alumina, ceria, or perovskite.
 5. A metal coated element for forming acurrent collector for an electrochemical device of the type thatincludes an ionically conductive electrolyte layer for gas generating,the invention comprising: spherical refractory material objects having aconductive coating of a precious metal; and the coated spherical objectshaving a desired diameter suitable for forming into a film; whereby thecoated spherical objects can be formed into a current conducting film.6. The invention of claim [claim Reference] wherein the precious metalis selected from the group consisting of silver, palladium, gold, orplatinum, or alloys of these metals with other metals.
 7. The inventionof claim [claim Reference] wherein the refractory material is a ceramic.8. The invention of claim [claim Reference] wherein the refractorymaterial is selected from the group consisting of zirconia, alumina,ceria, or perovskite.